(CNN) -- More than a quarter of young adults are unable to meet physical requirements to join the military, creating a potential threat to national security, a group of retired armed forces leaders said Tuesday.

"It's not drug abuse, it's not asthma, it's not flat feet -- by far the leading medical reason is being overweight or obese," said retired U.S. Air Force Lt. Gen. Norman Seip at a news conference.

About 27 percent of young adults are medically ineligible for the military, according to Mission: Readiness, a group of retired admirals, generals, and other senior military leaders.

Mission: Readiness' report, "Too Fat to Fight," said that 75 percent of young Americans between the ages of 17 to 24 do not qualify for the military because of failure to graduate [from high school], criminal records or physical problems. The study cited Department of Defense and health data.

Different branches of the military have their own policies, but they all measure strength, body fat, aerobic capacity, weight and height, Seip said.

A person must pass the physical fitness standards at the time he or she signs up for enlistment. These standards include sit-ups and push-ups.

"The logic is pretty obvious," said retired Army Brig. Gen. Clara Adams-Ender. "The troops need to be in excellent physical condition because of the demands of the important jobs they do in defense. Rigorous physical and mental standards are critical if we are to maintain the fighting readiness of our military."

The maximum weight depends on the person's sex, height and age. The Army allows a woman who is between 21-27 years of age, with the height of 5 feet 3 inches and no prior service, up to 137 pounds. A man between 21-27 years of age, height of 5 feet 8 inches and no prior service can weigh up to 186 pounds. The weight allotment increases with age and height.

The Army's body fat limit for women in the 21-27 age range, with no prior service is 32 percent body fat and for males, 26 percent.

Once enlisted, individuals also have to pass annual physical tests.

"We lose upwards to 12,000 young men and young women a year before they even finish up the first term of enlistment," Seip said. "That's another person, who has been recruited, trained and left because they're not able to maintain standards and can't pass the physical fitness test."

Retired officials said it's not about looking good in uniform, but ensuring the future health of the nation.

"We cannot wait until our young adults reach enlistment age to do something about it," said retired Navy Rear Adm. James Barnett Jr. "By that time, they may have already developed a chronic and lifelong weight problem."

Mission: Readiness urged Congress to pass a new childhood nutrition law to remove school junk food, improve nutritional standards and quality of school meals, and to open access to anti-obesity programs for children.

"If we do something about it, school can become a terrific environment for proper meals," Barnett said.

The retirees referred to a similar push military leaders made in 1945, when concerns about poor nutrition in potential recruits resulted in the creation of a national school lunch program.

The retired military leaders were joined by Sen. Richard Lugar of Indiana and Secretary of Agriculture Tom Vilsack.

"The reality that so many youngsters are not fit for military service is indeed a wake-up call for this country," Vilsack said.